A virus is a small particle which can infect other biological organisms. The term "virus" usually refers to those particles which infect eukaryotes (multi-celled organisms and many single-celled organisms), whilst "bacteriophage" or "phage" is used to describe those infecting prokaryotes (bacteria and bacteria-like organisms). Typically these particles carry a small amount of nucleic acid (either DNA or RNA) surrounded by some form of protective "coat" consisting of protein, or protein and lipid.

The protective coat normally also enables the infective process which can occur by a variety of different mechanisms. The practical upshot of all of these is that the host cell's replication machinery is hijacked to create more of the virus particles, hence completing the life cycle. Viruses are somewhere between being living and non-living. They can reproduce and show inheritance, but are reliant on the complex enzymes of their hosts, and in many ways can be treated like ordinary molecules (for instance, they can be crystalized). Whether or not they are "alive," it is clear that they are obligate parasites, and have no form which can reproduce independent of their host. Like most parasites they have a specific host range, sometimes specific to one species (or even limited cell types of one species) and sometimes more general.

Examples of diseases caused by viruses include the common cold, which is caused by a variety of related viruses; smallpox; AIDS, which is caused by HIV; and cold sores, which are caused by herpes simplex[?]. Recently it has been shown that cervical cancer is caused at least partly by papillomavirus (which causes papillomas, or warts), representing the first significant evidence in humans for a link between cancer and an infective agent.

Because they use the machinery of their host cells, viruses are difficult to kill. The most effective medical approaches to viral diseases, thus far, are vaccination to prevent infection, and drugs that treat the symptoms of viral infections. Patients often ask for antibiotics, which are useless against viruses, and their misuse against viral infections is one of the causes of antibiotic resistance in bacteria. That said, sometimes the prudent course of action is to begin a course of antibiotic treatment while waiting for test results to determine whether the patient's symptoms are caused by a virus or a bacterial infection.

The origin of viruses is not entirely clear, but the currently favoured explanation is that they are derived from their host organisms, originating from transferrable elements like plasmids or transposons. It has also been suggested that they may represent extremely reduced microbes, appeared separately in the primordial soup that gave rise to the first cells, or that the different sorts of viruses appeared through different mechanisms.

The word comes from the Latinvirus, referring to poison and other noxious things. Today it is used to describe the biological viruses discussed above and also as a metaphor for other parasitically-reproducing things, such as ideas. The term computer virus has become another well-defined sense of the word. The word virion[?] is used to refer to a single infective viral particle.

Despite frequent claims to the contrary, the only correct English plural of the word for any of these senses is viruses. The Latin word does not appear to have had a plural. Virii would be the plural of the word virius, and viri was the plural of the word vir, meaning man. See [1] (http://language.perl.com/misc/virus) for more on this.

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